1. Field of the Invention
The present invention relates to a spot killer circuit capable of effecting blanking in horizontal scanning or vertical scanning by using a first grid of a cathode ray tube comprised in a display unit.
2. Description of the Related Art
Generally, a display unit using a cathode ray tube is provided with a spot killer circuit for cutting off a luminescent spot on a display screen of the cathode ray tube when a power supply switch of the display unit is turned off.
An example of a conventional spot killer circuit is now described with reference to the accompanying drawings.
FIG. 1 is a circuit diagram of a conventional spot killer circuit. In FIG. 1, numeral 1 denotes a cathode ray tube, G1 a first grid of the cathode ray tube 1, and K a cathode of the cathode ray tube 1. A voltage of a negative voltage input terminal 2 is divided by first and second resistors R1 and R2 and the divided voltage at a junction therebetween is applied to the first grid G1 through a first diode D1 having an anode thereof connected to the first grid G1. Further, the first grid G1 is connected to a positive voltage input terminal 3 through a condenser C1.
The operation of the spot killer circuit constructed as shown in FIG. 1 is now described. In this case, it is assumed that the voltage of the negative voltage input terminal 2 is -100 V, a voltage of the positive voltage input terminal 3 is +100 V, and resistance values of the first and second resistors R1 and R2 are identical with each other. Further, a forward voltage drop of the first diode D1 is assumed to be OV.
When a power supply switch (not shown) is turned on, a voltage of -50 V produced by dividing the voltage of the negative voltage input terminal 2 by the first and second resistors R1 and R2 is applied to the first grid G1 and +100 V is applied to the positive voltage input terminal 3. At this time, the first condenser C1 is charged with a voltage of 150 V applied thereacross.
Then, when the power supply switch is turned off, the voltages of the negative voltage input terminal 2 and the positive voltage input terminal 3 are immediately reduced to zero. The potential of the cathode of the first diode D1 is also reduced to zero immediately. However, since the impedance of the first grid G1 is high, the negative voltage of the first condenser C1 is not discharged and the negative voltage side thereof is reduced to -150 V, so that the cathode ray tube 1 is cut off and a luminescent spot can be prevented from appearing on the display screen. The negative voltage across the first condenser C1 is gradually reduced to zero with a time constant defined by a product of a capacitance of the first condenser C1 and the impedance of the first grid G1, so that a luminescent spot can be prevented from remaining on the display screen of the cathode ray tube 1 by making the capacitance of the first condenser C1 preset to have a large value.
FIG. 2 is a circuit diagram of another circuit for effecting blanking in horizontal scanning or vertical scanning by using a first grid of a cathode ray tube 1. Like elements, which have appeared in the circuit diagram of FIG. 1 showing the conventional spot killer circuit, are designated by like numerals. FIG. 3 is a timing chart for explaining an operation of the circuit shown in FIG. 2. A blanking signal in horizontal scanning or vertical scanning is shown at (a) in FIG. 3, in which a period of time from time t1 to time t2 is a blanking period in horizontal scanning or vertical scanning.
A first grid G1 of a cathode ray tube 1 is supplied with a voltage of -50 V produced by dividing a voltage of a negative voltage input terminal 2 by first and second resistors R1 and R2. On the other hand, when a blanking signal for use in horizontal scanning or vertical scanning shown at (a) in FIG. 3 is supplied from a terminal 4 to the first grid G1 through a second condenser C2, the voltage of the first grid G1 assumes a waveform as shown at (b) in FIG. 3, since the impedance of the first grid G1 is high. As a result, since the voltage of the first grid G1 of the cathode ray tube 1 is reduced to -70 V during the blanking period from time t1 to time t2 in horizontal scanning or vertical scanning, the blanking in horizontal scanning or vertical scanning can be attained.
FIG. 4 is a circuit diagram which shows a combination of the circuit configurations shown in FIGS. 1 and 2, respectively, and which shows a circuit for effecting blanking in horizontal scanning or vertical scanning by using a first grid of a cathode ray tube 1 in the above-described conventional spot killer circuit. In FIG. 4, like elements, which have appeared in the circuit diagrams of FIGS. 1 and 2, respectively, are designated by like numerals.
The operation for preventing a luminescent spot from remaining on the display screen of the cathode ray tube can be performed satisfactorily in the same manner as described above. However, since the capacitance of the first condenser C1 is set to have a large value and hence the impedance of the circuit of the first grid G1 is decreased with respect to the blanking signal for use in horizontal scanning or vertical scanning which has been supplied to a terminal 4, a voltage signal supplied to the first grid G1 does not have such a waveform as shown at (b) in FIG. 3, but has a waveform similar to a sawtooth waveform as shown at (c) in FIG. 3 and has an amplitude not reaching -70 V, so that it becomes impossible to perform blanking in horizontal scanning or vertical scanning.